The interesting thing to me about all this angle business is how it helps set up both kouchi gari and ouchi gari while advancing (tori going forward).
Basically, while moving forward, you turn your right foot, toes inward (keeping your hips and shoulders facing forward).
As your left foot lifts, you pull yourself into uke (drawing his elbow toward your belly again). Your left foot move behind your right foot. Since the right foot was pre-turned, your hips and shoulders will turn slightly to match once you pick up your left foot. This turns both you and uke to the angle I talked about earlier.
From here, you can snag either ouchi or kouchi gari.
As it happens, this idea of pre-turning the right foot sits at the foundation of a lot of throws done while advancing, such as harai goshi (a favorite of mine, being long-legged), uchi mata, or tai otoshi (another favorite). It all depends on where I put that right foot. For these two inside sweeps, I tend to put it right between uke's feet (on his line).
Another advancing gari application that's popular around here is osoto. This time, tori pre-turns his left foot, and his toes are pointed outward.
As you pick up your right foot, and as uke picks up his left, you do the elbow draw as your hips and shoulder naturally turn to line up with your left foot. This will turn uke as well, until you find yourself in the same perpendicular position.
What about kosoto advancing? Well, I'll have to get to that, plus a few other kosoto ideas next.